The present invention relates to a semi-transmission/reflection-type liquid crystal display device that displays an image by selectively using transmission light and reflection light.
Being thin, light, and low in power consumption, liquid crystal display devices in which a liquid crystal display panel is used as an image generating means are employed as display devices in a wide variety of electronic equipment, such as notebook-sized personal computers, word processors, electronic notes, cellular phones, and camera-incorporated video recorders.
In contrast to CRTs and plasma display panels, liquid crystal display panels display an image by controlling the quantity of light that has entered the panel from the outside instead of emitting light by itself. When equipped with color filters of plural colors as light control elements, liquid crystal display panels can display a color image of multiple colors.
Liquid crystal display devices that are used most commonly at present are transmission-type devices in which an illumination light source, called a backlight that uses a fluorescent tube, is provided on the back side of a liquid crystal panel, and an image is displayed by controlling the quantity of light (part of light emitted from the backlight) that passes through the liquid crystal panel.
However, in such transmission-type liquid crystal display devices, the power consumption of the backlight accounts for approximately one half of the total power consumption. This is a major factor in shortening the usable time in a case where portable electronic apparatuses as exemplified above are of a battery-driven type. Transmission-type liquid crystal display devices have another problem, in that, when they are used in a bright outdoor environment, for example, ambient light is reflected by the surface of the display area and a displayed image becomes hard to recognize.
Among liquid crystal display devices that are always used in a carried state in a bright environment such as found outdoors, there are reflection type liquid crystal display devices that usually do not use a backlight, but are equipped with a reflection plate, and control the quantity of reflection light (part of ambient light) with the liquid crystal layer. An example of such liquid crystal display devices is one that performs both transmission-type display and reflection-type display using a semitransparent reflecting film (e.g., Japanese Unexamined Patent Publication No. Hei. 7-333598).
Another example of the above type of liquid crystal display device is one in which each pixel electrode is composed of two regions that comprise a reflection region and a transmission region (e.g., Japanese Unexamined Patent Publication No. Hei. 7-333598).
However, in the above conventional liquid crystal display devices, the display quality varies depending on the use environment (light-source-related environment). That is, a display that is performed by using reflection light (reflection light mode) and a display that is performed by using transmission light (transmission light mode) having different contrast ratios. Further, a coloration phenomenon may occur in black-and-white display and hue deviation may occur in color display. The difference in contrast ratio is caused by the fact that the black display luminance (off transmittance) and the white display luminance (on transmittance) are different between the case where reflection light is used and the case where transmission light is used. This phenomenon lowers the legibility of a displayed image. The hue deviation is a phenomenon that the hue shifts to the bluish side particularly in the case where transmission light is used. This deteriorates the color reproduction performance.
The present invention has been made to solve the above problems in the art, and an object of the invention is therefore to provide a liquid crystal display device which is capable of image display with a large contrast ratio in both a transmission display mode and a reflection display mode, as well as being capable of good color display in both display modes.
To attain the above object, the invention provides the following configuration. A liquid crystal is interposed between a first transparent substrate having first electrodes and a second transparent substrate having other electrodes that are opposed to the first electrodes. Pixel regions are formed at portions where the first electrodes and the other electrodes are opposed to each other. A semitransparent reflecting film (or an opaque reflecting film) is formed between the first transparent substrate and the first electrodes. The semitransparent reflecting film is formed with light transmission apertures in each pixel region. Or, the semitransparent reflecting film is formed with slits at positions corresponding to the gaps between adjacent pixel regions. With the above measure, part of the illumination light that comes from the first transparent substrate side is introduced to the liquid crystal through the light transmission apertures or the slits.
An opaque reflecting film may be formed instead of a semitransparent reflecting film. In the case of an opaque reflecting film, the combination of the opaque reflecting film and the light transmission apertures serves as the above-mentioned xe2x80x9csemitransparent reflecting film.xe2x80x9d In the case of the semitransparent reflecting film, the semitransparent reflecting film itself and the light transmission apertures serve as the above-mentioned xe2x80x9csemitransparent reflecting film.xe2x80x9d The same interpretation applies to the term xe2x80x9csemitransparent reflecting filmxe2x80x9d that will be used in the following description and embodiments.
With the above configuration, in the transmission light mode, part of the light that comes from the outside of the first transparent substrate is output from the second transparent substrate after passing through color filters. Therefore, not only is the legibility of a display image improved, but also a hue deviation in transmission light is decreased, which improves the color reproduction performance.
Where the semitransparent reflecting film also occupies the portions corresponding to the gaps between the adjacent pixel regions, the contrast in the transmission light mode is increased.
The peripheral portions of adjacent ones of color filter layers that are formed between the second transparent substrate and the other electrodes may overlap with each other to provide a light shield function. Since the overlapping portions of the color filter layers serve as light shield layers, the contrast is increased.
A light absorption film may be formed under the slits that are formed on the side of the first transparent substrate, or the slits may be charged with a light absorption film. This prevents color mixture between adjacent pixels and hence increases the contrast.
A semi-transmission/reflection-type liquid crystal display device is constructed by disposing an illumination light source on the back side of the first transparent substrate of the above liquid crystal display panel. In an environment where the brightness is sufficiently high, the semi-transmission/reflection-type liquid crystal display device is used in the reflection light mode by turning off the illumination light source. In a dark environment, it is used in the transmission light mode by turning on the illumination light source. The color reproduction performance is improved in either mode.
In the semi-transmission/reflection-type liquid crystal display device using the above liquid crystal display panel, an upper polarizer and a lower polarizer are formed on the display screen side (i.e., the second transparent substrate side) of the liquid crystal display panel and on the side opposite to it (i.e., the first transparent substrate side), respectively, and their optical absorption axes (polarizing axes) are set approximately perpendicular to each other.
A first alignment layer is formed on the inside surface of the substrate (first transparent substrate) provided on the illumination light incidence side (in the transmission light mode), and the alignment axis of the first alignment layer and the absorption axis of the lower polarizer are set approximately parallel with each other. The black display luminance (off transmittance) that is obtained when black display voltages are applied to the pixel electrodes of the liquid crystal display panel is made low and the white display luminance (on transmittance) that is obtained when white display voltage are applied to the pixel electrodes is made high, whereby the contrast ratio of a display image is increased irrespective of the display mode.
A first upper phase plate and a second upper phase plate are formed on the substrate provided on the display screen side of the liquid crystal display panel and their drawing axes are deviated from each other by about 30xc2x0(30xc2x0xc2x120xc2x0), whereby light that has passed through the liquid crystal layer is converted into approximately linearly polarized light. This prevents a coloration phenomenon in black-and-white display and hue deviation in color display (neutralization of display color) and thereby enables high-quality color reproduction that is free of hue deviation.
Typical configurations according to the invention are as follows.
(1) A liquid crystal display device comprising:
a first transparent substrate having a plurality of first electrodes;
a second transparent substrate having a plurality of second electrodes (other electrodes) that are opposed to the first electrodes;
a liquid crystal interposed between the first transparent substrate and the second transparent substrate;
an illumination light source disposed on the back side of the first transparent substrate;
pixel regions formed at portions where the first electrodes and the second electrodes are opposed to each other; and
a reflecting film formed between the first transparent substrate and the first electrodes, the reflecting film having one or a plurality of light transmission apertures in each pixel region and not having slits at positions corresponding to gaps between adjacent ones of the pixel regions.
(2) In configuration (1), color filter layers are further provided between the second transparent substrate and the second electrodes, and peripheral portions of adjacent ones of the color filter layers overlap with each other at positions corresponding to the gaps between adjacent ones of the pixel regions.
(3) A liquid crystal display device comprising:
a first transparent substrate having a plurality of first electrodes;
a second transparent substrate having a plurality of second electrodes that are opposed to the first electrodes;
a liquid crystal interposed between the first transparent substrate and the second transparent substrate;
illumination light source disposed on the back side of the first transparent substrate;
pixel regions formed at portions where the first electrodes and the second electrodes are opposed to each other;
a reflecting film formed between the first transparent substrate and the first electrodes, the reflecting film having one or a plurality of light transmission apertures in each pixel region and slits at positions corresponding to gaps between adjacent ones of the pixel regions; and
a light absorption film formed between the first transparent substrate and the reflecting film at positions corresponding to the slits.
(4) A liquid crystal display device comprising:
a first transparent substrate having a plurality of first electrodes;
a second transparent substrate having a plurality of second electrodes that are opposed to the first electrodes;
a liquid crystal interposed between the first transparent substrate and the second transparent substrate;
illumination light source disposed on the back side of the first transparent substrate;
pixel regions formed at portions where the first electrodes and the second electrodes are opposed to each other;
a reflecting film formed between the first transparent substrate and the first electrodes, the reflecting film having one or a plurality of light transmission apertures in each pixel region and slits at positions corresponding to gaps between adjacent ones of the pixel regions; and
a light absorption film with which the slits are charged.
(5) In each of configurations (1)-(4), the reflecting film is an opaque reflecting film.
(6) In each of configurations (1)-(4), the reflecting film is a semitransparent reflecting film.
(7) A liquid crystal is interposed between a first transparent substrate having first electrodes and a second transparent substrate having other electrodes that are opposed to the first electrodes. Pixel regions are formed at portions where the first electrodes and the other electrodes are opposed to each other. A semitransparent reflecting film that is formed with light transmission apertures in each pixel region is formed between the first transparent substrate and the first electrodes.
(8) Color filter layers in which the peripheral portions of the adjacent color filter layers overlap with each other are provided between the first transparent substrate and the first electrodes and the overlapping peripheral portions serve as light shield films located at positions corresponding to the gaps between adjacent pixel regions.
(9) A liquid crystal is interposed between a first transparent substrate having first electrodes and a second transparent substrate having other electrodes that are opposed to the first electrodes. Pixel regions are formed at portions where the first electrodes and the other electrodes are opposed to each other. A semitransparent reflecting film that is formed with light transmission apertures on each pixel region and slits extending along the peripheries of the pixel regions are formed between the first transparent substrate and the first electrodes.
(10) A light absorption film is formed under the slits (i.e., on the side of the first transparent substrate), or the slits are charged with a light absorption film.
(11) A semi-transmission/reflection-type liquid crystal display device is constructed by disposing an illumination light source on the back side of the first transparent substrate of the liquid crystal display panel of each of configurations (7)-(10).
(12) In configuration (11), an upper polarizer and a lower polarizer whose absorption axes (polarizing axes) are set approximately perpendicular to each other are formed on the surface on the second transparent substrate side and the surface on the first transparent substrate side, respectively. A first alignment layer and a second alignment layer are formed at the boundaries between the liquid crystal and the inside surfaces of the first transparent substrate and the second transparent substrate, respectively, and the alignment axis of the first alignment layer and the absorption axis of the lower polarizer are set approximately parallel with each other. A first upper phase plate and a second upper phase plate whose drawing axes are deviated from each other by about 30xc2x0 (30xc2x0xc2x120xc2x0) are formed on the outside surface of the second transparent substrate.
With the above configuration, the black display luminance (off transmittance) that is obtained when a black display voltage is applied to the first pixel electrodes and the other pixel electrodes of the liquid crystal display panel is made low and the white display luminance (on transmittance) that is obtained when white display voltages are applied to the pixel electrodes is made high, whereby the contrast ratio of a display image is increased irrespective of the display mode. Therefore, the legibility is improved and a high-quality liquid crystal display panel can be realized.
A diffusion layer is provided between the first upper phase plate and the second upper phase plate. A known diffusion sheet may be used as the diffusion layer. However, if the diffusion layer is formed by mixing light diffusing particles into an adhesive for bonding the first and second upper phase plates to each other, the optical loss is minimized and bright display can thereby be attained.
Each of the above configurations makes it possible to produce a bright, clear image having a large contrast ratio or a high-quality color image that is free of hue deviation in both an environment having bright ambient light and a dark environment by selectively,using transmission light and reflection light.
The invention is not limited to the above configurations or the embodiments described below. It goes without saying that various modifications are possible without departing from the spirit and scope of the invention.